Leadless semiconductor package and method of manufacture

ABSTRACT

A leadless semiconductor package includes a package body on a leadframe that includes a die paddle and a plurality of bond pads, none of which extend as far as a lateral face of the body. During manufacture of the package, molding compound is deposited over a face of the leadframe on which the die paddle and bond pads are positioned. After the molding compound is cured, a back side of the leadframe is etched to isolate the die paddle and bond pads, back surfaces of which remain exposed at a back face of the body. During manufacture of the leadframe, a parent substrate is etched to define the die paddle and a plurality of bond pads on one side of the substrate and a plurality of cavities on the opposite face.

BACKGROUND OF THE INVENTION

1. Technical Field

The present disclosure relates to semiconductor packaging, and inparticular to leadframe-based packages, to leadframes employed in suchpackages, and most particularly to flat, leadless type packages andleadframes, such as, e.g., QFN packages.

2. Description of the Related Art

While significant advances have been made in semiconductor packaging,including the development of a very large number of packaging types, themajority of semiconductor devices still employ leadframe based packages.This is due to a number of reasons. In particular, leadframe packagesare relatively inexpensive to produce, are well known and understood,the tooling is already in hand or readily available, etc. Additionally,leadframe packages have some advantages over many other packages,including having better thermal transmission characteristics, and morebeing more robust. As industry demand moves toward smaller packages andhigher contact density, leadframe technology continues to evolve, sothat even within the general category of leadframe packages, there aremany different types. However, leadframe-based packages have at leastone element in common: they all have a leadframe. Leadframes are usuallystamped or chemically milled from thin metal strips, which are generallyformed in rolls. Typically, a leadframe includes a die paddle on which asemiconductor die is mounted, and a plurality of contact pads that areplaced in electrical contact with respective bond pads of the leadframe.

Leadframes are machined from a roll, i.e., strip, of metal foil, usuallycopper, which can be plated to prevent oxidation that might otherwiseinterfere with making good electrical contact. Chemical milling istypically used to form leadframes that have very fine details, such aswith near chip-scale frames and frames that employ a large number ofleads.

FIG. 1 is a plan view of a portion of an exemplary leadframe strip 100for a quad flat no-lead (QFN) semiconductor package. The strip 100 ofFIG. 1 includes a plurality of die paddles 102, and a plurality of leads104 associated with each die paddle. Support stringers 106 extendbetween a support frame 108 and the die paddles 102, and the bond padsare attached directly to the frame. During the packaging process asemiconductor material die is attached to the die paddle 102, usuallyvia a thermally conductive adhesive. Bond wires are attached at one endto contact pads of the semiconductor die, and to respective ones of theleads 104 at the other end, thereby placing the leads in electricalcontact with the contact pads of the semiconductor die. Prior toattaching the semiconductor dies, a carrier tape is adhered to the backside of the leadframe strip 100, and after the wire bonding step, amolding compound is applied over the semiconductor die and bonding wiresto form a protective package body, the approximate dimensions of whichare indicated by the dashed lines 110 of FIG. 1. The carrier tapeprevents the molding compound from covering the back side of theleadframe, so that the back sides of the die paddles 102 and leads 104are exposed for contact with a circuit board. After the molding compoundhas been cured, the carrier tape is removed and the packages are punchedfrom the leadframe strip. In the punching process, portions of theleadframe that protrude from each package are trimmed very close to theface of the package. The exposed die paddle on the back side of eachpackage permits a good thermal contact between the semiconductor die anda circuit board, while the exposed leads 104 enable mounting of thepackage to contacts on the circuit board that are positioned inside thefootprint of the package, which reduces the surface area occupied by thepackage on the circuit board.

There is continual industry pressure to reduce the thickness ofsemiconductor packages, especially for use in consumer electronicdevices such as cell phones and PDA devices. In some cases, OEMmanufacturers require that a package height be no more than 400 μm, andit is likely that even thinner packages will be required in the future.One problem associated with leadframe-based packages is that there is aminimum thickness requirement for leadframe material. Typically,leadframe strips cannot be much less than about 100 μm, because thinnermaterial becomes virtually impossible to handle without damage. It lacksthe strength and rigidity to withstand normal handling and the processesto which it is subjected during typical manufacturing operations. To theextent that special tooling and procedures could be devised tosuccessfully handle thinner material, this would represent a significantadditional expense and would eliminate one of the primary advantages ofleadframes over other packaging formats, i.e., the ability of manyexisting automated systems to perform some or all of the processingsteps required for more advanced package designs.

FIG. 2 is a simple diagrammatic side view of a portion of a leadframestrip 120 according to known art. A carrier strip 122 is provided, onwhich separate die paddles 124 and leads 126 are formed by a metaldeposition process. The carrier strip acts as a support substrate sothat the die paddles and bond pads can be made much thinner than thenominal 100 μm limit of the traditional punched or chemically milledleadframe strips. Additionally, because the elements of the leadframeare individually formed, there is no requirement for a support frame,which permits greater freedom to the designer because there is norequirement that every feature have an unobstructed path to theperimeter. Furthermore, because the features are not required to beself-supporting, elements that are very thin or closely spaced can beformed, where similar elements on a traditional leadframe would bend orshort against other elements. Unfortunately, the processes necessary tomanufacture leadframes by metal deposition are expensive and timeconsuming.

BRIEF SUMMARY

According to one embodiment, a leadless semiconductor package isprovided, including a package body on a leadframe that includes a diepaddle and a plurality of bond pads, all of which have a back surfaceexposed at the back side of the body, but none of which extend as far asa lateral face of the body. The die paddle and bond pads can be plated,on their respective front faces and/or back faces, with a protectivemetal layer.

During manufacture of the package, molding compound is deposited over afront face of the leadframe on which the die paddle and bond pads arepositioned. The face of the leadframe is substantially continuous andwithout perforations for at least the lateral dimensions of the body.After the molding compound is cured, a back side of the leadframe isetched to isolate the die paddle and bond pads, back surfaces of whichremain exposed at a back face of the body after the etching.

During manufacture of the leadframe, a parent substrate is etched on afirst face to define elements of the leadframe such as, e.g., the diepaddle and the plurality of bond pads. The substrate is also etched on asecond face, opposite the first face, to define a plurality of cavitieson the second face, each positioned directly opposite a correspondingone of the elements on the first face, and each sized and shaped tosubstantially match the size and shape of the corresponding element. Aprotective metal layer is plated onto the surfaces of the elements ofthe lead frame and onto an inner wall of each of the cavities. The layerof metal plated onto the inner wall of each of the cavities acts as anetch stop to protect the corresponding element during the etch performedas part of the packaging process.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a plan view of a portion of an exemplary leadframe strip for aflat no-lead semiconductor package, according to known art.

FIG. 2 is a simple diagrammatic side view of a portion of anotherleadframe strip according to known art.

FIG. 3 is a plan view of a leadframe-based semiconductor packageaccording to one embodiment.

FIG. 4 is a cross-sectional view of the semiconductor package of FIG. 3taken along lines 4-4 of FIG. 3.

FIG. 5 is a plan view of a leadframe-based semiconductor packageaccording to another embodiment.

FIG. 6 is a cross-sectional view of the semiconductor package of FIG. 5,taken along lines 6-6 of FIG. 5.

FIG. 7 is a plan view of a portion of a leadframe strip according to oneembodiment.

FIG. 8 is a perspective view of a portion of the leadframe strip of FIG.7, showing the front side of the leadframe, with a smaller portion cutaway to show elements of the back side.

FIGS. 9A-9C are cross-sectional diagrams of the leadframe at respectivestages of manufacture, according to one embodiment, as viewed from aposition corresponding to the lines 9A-9A of FIG. 8.

FIGS. 10A-10D are diagrams showing respective stages of the assembly ofsemiconductor packages according to one embodiment, using the leadframestrip described with reference to FIGS. 7-9C. FIGS. 10A, 10B and 10D arecross-sectional views of an assembly of semiconductor packages takenalong a plane corresponding to the plane defined in FIG. 8 by lines9A-9A, but extended through two leadframes. FIG. 10C is an enlarged viewof a small portion of the assembly of FIG. 10B, as defined by the ovalin FIG. 10B that is marked 100.

DETAILED DESCRIPTION

Turning now to FIG. 3, a plan view is provided of a leadframe-basedsemiconductor package 10 according to an embodiment. The package 10 isencapsulated in a molding compound body 12 that is shown as thoughtransparent in order to show the internal structure. The package 10 isin a quad flat pack, no lead (QFN) configuration, and includes asemiconductor die 14 with a plurality of contact pads 17, a plurality ofbond pads 16, and a plurality of bond wires 18 coupled betweenrespective pairs of the bond pads and the contact pads.

FIG. 4 is a cross-sectional view of the semiconductor package 10 takenalong lines 4-4 of FIG. 3. Portions of a leadframe 21 are visible,including a die paddle 24 on which the semiconductor die 14 is mounted,and also including a plurality of bond pads 16. The die paddle 24 andbond pads 26 are formed from a common sheet of metal, preferably copperor a copper alloy. An upper plating 26 and a lower plating 22 arepositioned on each of the bond pads 16, while the die paddle 24 includesan upper plating 28 and a lower plating 23. As discussed with referenceto the prior art devices of FIGS. 1 and 2, the leadframe elements,including the die paddle 24 and the bond pads 16, are exposed andelectrically accessible at the back side of the package 10.

FIG. 5 is a plan view of a leadframe-based semiconductor package 40according to another embodiment. The package 40 is substantially similarto the package 10 described with reference to FIGS. 3 and 4, except thatit also includes a contact ring 44 extending about the semiconductordevice 14 and separated therefrom by a gap 42 which electricallyisolates the contact ring from the device 14. The contact ring 44 may beprovided, for example, where a number of the contact pads 17 on thesemiconductor device 14 are required to be coupled to a common node,such as for a positive or negative power supply, or ground, etc., orwhere multiple contacts on an underlying circuit board are required tobe coupled to such a common node. The package 40 includes a leadframe41, partially shown in the cross-sectional view of FIG. 6, which alsoshows that the contact ring 44 includes an upper plating 46 and a lowerplating 52.

It can be seen, with reference to FIG. 5, that the leadframe elements ofthe package 40 do not include features commonly associated withleadframe strips. For example, the die paddle 24 does not includesupport stringers, like those shown in the prior art device of FIG. 1,to support the die paddle as part of the leadframe strip until a packageis formed around it. Furthermore, the die paddle 24 is completelysurrounded by the contact ring 44, so it would be impossible to haveprovided support stringers extending from the die paddle to a supportframe while keeping the support ring electrically isolated from the diepaddle. Lastly, the bond pads 16 do not include portions extending tothe perimeter of the package as would normally be the case.

FIG. 7 is a plan view of a portion of a leadframe strip 60 according toone embodiment. Leadframe features of a package similar to the package40 described with reference to FIGS. 5 and 6 are shown. Also shown indashed lines are cut lines 25 along which semiconductor packages formedon the leadframe strip 60 will eventually be cut, and whichapproximately define the size and shape of the finished packages. Theleadframe strip 60 comprises a metal substrate 62 that is substantiallycontinuous and unbroken, at least within the areas of the packages, asdefined by the cut lines 25. This will be explained in detail hereafter.The leadframe strip 60 includes perforations 64 extending along the longedges of the strip to facilitate processing of the leadframe strip andby machinery used to make the strip and form semiconductor packagesthereon.

The specific elements shown in FIG. 7 are merely exemplary. In practice,a leadframe strip according to the principles of the invention caninclude a wide range of features, including multiple rows of bond pads,multiple concentric contact rings, etc. Additionally, while FIG. 7 showsa leadframe strip with rows of two frames per row, the actual number offrames per row, as well as the configuration and location ofperforations and fiducials for proper machine handling and processingdepend on the design and size of the particular package, the size of theparent roll of material, the makes and models of the machinery that willbe used to process the strip, etc.

Turning now to FIG. 8, a perspective view of a portion of the leadframestrip 60 is provided, showing the front side of the leadframe, with asmaller portion cut away to show elements of the back side of the strip.The leadframe strip 60 is preferably made from a parent roll of copperor copper alloy material. The substrate 62 is formed so that thefeatures of the leadframe extend above the surrounding surface 39 of thesubstrate, i.e., away from the lower portions of the front face. Thus,for example, a channel or trench 38 is part of the lower face 39. Thechannel 38 defines a separation between the contact ring 44 and the diepaddle 24. Cavities are formed on the back side of the substrate 62 inpositions directly opposite the leadframe elements formed on the frontside: a cavity 32 is positioned opposite each of the bond pads 16; agroove or channel 34 is positioned opposite the contact ring 44, and acavity 36 is positioned opposite the die paddle 24. Front surfaces ofleadframe elements on the front face, including the die paddle 24, thecontact ring 44, and the bond pads 16, are provided with a layer ofmetal plated thereon. Inside walls of the cavities 32, 34, 36 formed inthe back side of the substrate are also plated.

The metal used to plate the elements of the leadframe can be anyappropriate formulation, but is preferably resistant to oxidation andcorrosion, in order to provide a reliable contact surface for thewirebonding process. According to one embodiment, the plating includes alayer of nickel, a layer of palladium, and a layer of gold. Totalthickness of the plating will usually not exceed about 5 μm, and ispreferably less than about 2 μm. Most of that thickness is made up ofthe nickel layer, with the layers of palladium and gold being relativelymuch thinner.

The metal plated in the cavities on the back side can be the samemetals, or can be different metals. During the packaging process, theplated metal in the cavities will be used as an etch resist, then laterduring assembly of an electronic device that incorporates the completedpackage, the same metal will form contact surfaces for establishingelectrical contact between the semiconductor die and contacts of acircuit board.

Generically, leadframe features formed in the manner described above canbe referred to as lands, while the surrounding surface can be referredto as the lower face. This is not to be confused with the back side orback face of the substrate, which is on the opposite side from thelands. The lower face 39 is part of the front face of the substrate 62.

A process for manufacturing the leadframe 60, according to oneembodiment, will be described in more detail with reference to FIGS.9A-9C. Thereafter, a process for forming a semiconductor package on theleadframe, according to an embodiment, will be described with referenceto FIGS. 10A-10D.

FIGS. 9A-9C are cross-sectional diagrams of a leadframe at respectivestages of manufacture, according to one embodiment, as viewed from aposition corresponding to the lines 9A-9A of FIG. 8. The process beginswith a strip 64 of copper having a thickness T1 of about 100 μm. Aftercleaning to remove oxides that normally form on copper, a layer of metal65 is plated onto a first side of the strip—hereafter the front side. Alayer of etch resist 66 is then deposited over the metal layer 65 andpatterned to protect the portions of the parent strip that will becomelands on the front side of the leadframe. A second layer of etch resist68 is deposited on the back side of the strip and patterned to exposethe areas of the back side where the cavities are to be formed. This isthe stage in the process that is shown in FIG. 9A.

As shown in FIG. 9B, the front side of the strip 64 is etched to removethe plated metal 65 that is not protected by the etch resist layer 66,then both sides are etched to remove a selected thickness of the parentstrip 64 to form cavities 32, 34, and 36 on the back side, and the lowerface 39, including the trench 38, on the front side. In the embodimentshown in FIG. 9B, each side is etched to a depth of about half the totalthickness of the strip 64. The lateral dimensions, shape, and positionof each of the cavities 32, 34, 36 on the back side are selected so asto be aligned with and slightly smaller, in all lateral dimensions, thanthe corresponding land on the front side of the strip. In this way, evenif each side is etched to half the total thickness, a thin web ofmaterial 72 will remain between the deepest part of each cavity and anypart of the front surface, so that no complete perforations are made inthe substrate. A web 72 extends around the entire perimeter of andsupports each feature formed on the front side of the substrate.

Following the etch step, the back side of the substrate is plated. In anelectroplating operation, the material to be plated must be electricallyconductive. Because the back side of the strip 64 is entirely covered bya layer of non-conductive etch resist except over the newly formedcavities 32, 34, and 36, only the inner walls of the cavities are platedin the operation. If necessary, the front side of the strip can betemporarily covered to prevent plating on that side. Finally, the etchresist layers on both sides of the strip 64 are removed from the strip,leaving the completed leadframe, shown in close detail in FIG. 9C.

While the cavities are disclosed as being slightly smaller than thecorresponding feature on the opposite face of the substrate, accordingto an alternative embodiment, the cavities can be identical in size andshape to the corresponding feature. in such an embodiment, the etch stepis controlled to be terminated before the etch process extends half thetotal thickness from each side. instead, the process is controlled toterminate when between about 5%-10% of the thickness remains between theetches. In this way, a supporting structure similar to the web 72 willremain between the cavities and the lower face 39. One advantage to thisembodiment is that the masks used to form the resist layers 66 and 68can be mirror images of each other, which is simpler and less expensiveto produce than a mask with features that are sized differently. In somecases it may even be possible to use the same mask on both sides. Ofcourse, it would be necessary to use a negative-acting photoresist onone side, and a positive acting resist on the other.

FIGS. 10A-10D are diagrams showing respective stages of the assembly ofsemiconductor packages according to one embodiment, using the leadframestrip 60 described with reference to FIGS. 7-9C. FIGS. 10A, 10B and 10Dare cross-sectional views of an assembly of semiconductor packages 70taken along a plane corresponding to the plane defined in FIG. 8 bylines 9A-9A, but extended through two leadframes. FIG. 10C is anenlarged view of a small portion of the assembly 70, as defined by theoval in FIG. 10B that is marked 10C.

First, as shown in FIG. 10A, semiconductor dice 14 are adhered torespective die paddles 24, and contact pads of the dice are wirebondedto the appropriate bondpads 16 or contact rings 44. These processes arevery well known and understood in the art, and so will not be describedin detail.

Moving to FIG. 10B, a molding compound layer 33 is then deposited overthe leadframe 60 and cured, encapsulating the semiconductor dice 14 andbond wires 18. The molding compound 33 is entirely contained, on theback side, by the leadframe strip 60, so an adhesive carrier, like thatdescribed in the background is not required. Once the molding compoundis cured, the back side of the leadframe strip 60 is etched toselectively remove portions thereof. Turning again briefly to FIG. 9C,an etch process that is formulated to dissolve copper, but not the metalplated into the cavities 32, 34, 36, when applied from the back side ofthe strip, will remove the material of the substrate that lies betweenthe cavities, until the process reaches the lower face 39 of the frontside. Bearing in mind that molding compound will have been deposited andcured on the front side, when the etch reaches the lower face, it willsubstantially stop, having only the narrow webs 72 against which tocontinue acting, and the etching process is terminated before it canhave an appreciable effect on the lands of the leadframe, which areencapsulated in the molding compound 33. With the etching of the backside of the leadframe strip 60 to remove all but the small portions ofthe leadframes that are protected by the metal deposits, the elements ofthe leadframes are now electrically isolated from each other exceptwhere they are coupled by the bond wires 18.

Once the etching process is complete, the portions of the plating thatwere on the sidewalls of the cavities will be unsupported, as shown inthe detail of FIG. 10C. Bearing in mind that in practice the layers ofplated metal are only microns thick, the unsupported portions are littlemore than fragile burrs surrounding the contact surfaces on the backside of the package. The greatest danger is that pieces may break awayand create short circuits or otherwise interfere with proper operationof the device. Accordingly, the assembly is pressure washed by as jet ofhigh-pressure water, which quickly breaks off and removes all of theloose material, leaving the plated surfaces 22, 46, and 23 on the backside of the package.

Finally, as shown in FIG. 10D, the assembly 70 is cut into separatepackages 40, which are tested and marked.

It will be recalled that in the first etch process, described withreference to FIGS. 9B and 9C, each side of the substrate is etched to adepth of about half the total thickness of the substrate. Thus, thedistance from the lower face 39 to the front faces of the lands is equalto half the thickness of the original substrate, as is the depth of thecavities formed in the back side. Therefore, following the second etchstep, described with reference to FIGS. 10B and 10C, half of theoriginal thickness will have been removed. Assuming an originalthickness of 100 μm, the remaining nominal thickness of the die paddle24 and bond pads 16 is only 50 μm, which is also half of the minimumpractical thickness of traditional leadframes, as explained in thebackground. Furthermore, by adjusting the time or chemistry of theetches performed on each side of the substrate in the first etchprocess, the relative depths can also be adjusted, so that the finishedthickness of the leadframe is fully selectable. Even without changingthe original thickness of the substrate, the finished thickness can becontrolled to be anywhere from as thin as around 10 or 20 μm to as thickas about 60 or 70 μm. The upper and lower limits will in part bedetermined by the kinds and sizes of leadframe elements formed on thesubstrate.

It should be born in mind, for example, that at any given point, thesubstrate of the leadframe 60 of FIG. 8 is only about 50 μm inthickness. However, this is acceptable here, where a traditionalleadframe of a similar thickness would be far too weak, because, first,in the disclosed embodiments there are no perforations and no separateelements that must retain position and shape while only supported bythin and often cantilevered connections to a support frame; and second,because the alternating etched regions on the opposing sides of thesubstrate behave like corrugations in a piece of sheet metal, whichserve to stiffen the metal to a condition that is far more rigid thanthe same thickness of sheet metal would be if completely flat.

As previously mentioned, a leadframe configured according to theprinciples disclosed above is not limited by the need to have all theelements coupled to a support frame. Instead, of being supported by aframe that lies in the same plane as the elements, but outside thefinished dimensions of a putative package, the elements of the variousembodiments are supported from structure that is much closer, lying aplane that defines a face of the intended package. One resultingdistinction is that, unless there is a practical and beneficial reason,a semiconductor package formed in accordance with an embodiment will nothave any portion of the original leadframe visible or exposed around itslateral perimeter. This is in contrast to most leadframe-based devises,including the one described in the background with reference to FIG. 1.While the prior art leadframe described with reference to FIG. 2 issimilar to the disclosed embodiments in this respect, it suffers from anumber of other deficiencies, including the cost to produce theleadframe, and also the cost and complexity to use in packagingsemiconductor devices.

While a particular series of process steps has been disclosed formanufacturing a leadframe, there are many different known processesrelated to patterning and etching, many of which are interchangeable andcan be selected, often merely on the basis of preference. Thus, anendless variety of different ways can be devised for producing, forexample, the leadframe of FIG. 8, based on, for example, differentorders of operation, different masking schemes, selection of resist andetch chemistries, le requirements, etc. Accordingly, the claims are notlimited to any particular process or steps of a process except to theextent that the process or steps are explicitly recited, and then onlyin the claims in which they are recited, and claims that dependtherefrom.

Additionally, while the preferred method of manufacture is by chemicalmachining, the substrate of a leadframe can also be stamped to formlands on the front face and corresponding cavities on the back face. Theparent sheet is plated on both faces prior to being stamped, and theback side is also coated with an etch resist. The back side is subjectedto a brief abrasion after stamping, which removes the plating and resiston the back surface, without removing them from the cavities. After themolding compound is deposited and cured, and the back of the leadframeis etched, the process will proceed as described above, except that theplating on the front face will remain. It is then necessary to performanother etch to remove the plating, while the plating in the cavitieswill be largely protected by the etch resist remaining in the cavities,although the burrs extending around the sidewalls of the cavities willbe removed as well, eliminating the need for the pressure washing step.Following the etch process, the etch resist is removed using theappropriate solvents.

As used in the specification and claims, the term front is used to referto the side of a leadframe that would face away from a circuit board ifthe leadframe were in a package mounted to the circuit board accordingto its design. Conversely, back is used to refer to the side or faceopposite the front side and facing the circuit board. Use of front andback with reference to other related elements are to be construedaccordingly.

The term land is used to refer to the features of a leadframe thatextend above the surrounding surface on the front face, and the termlower face is used to refer to a part of the front face of a leadframethat extends around the lands but lies closer to the back face.

The term lateral is used to refer to directions or dimensions extendingin a plane that lies substantially parallel to the front and back facesof a leadframe.

The term coupled, as used in the claims, includes within its scopeindirect coupling, such as when two elements are coupled with one ormore intervening elements even where no intervening elements arerecited. For example, where a claim recites a fluid output of a firstheat exchanger coupled to a fluid input of a second heat exchanger, thislanguage also reads on embodiments in which fluid passes from a firstheat exchanger through a turbine before entering a second heatexchanger.

The term over is used in the specification and claims to refer to therelative positions of two or more elements with respect to a thirdelement, although the third element may be implied by the context. Theterm on is used to refer to a physical relationship between twoelements. Neither term should be construed as requiring direct physicalcontact between the elements, nor should they be construed as indicatingany particular orientation, either absolute, or with respect to thethird element. So, for example, if a claim recites a second layerpositioned over a first layer on a substrate, this phrase indicates thatthe second layer is coupled to the substrate and that the first layer isbetween the second layer and the substrate. It does not indicate thatthe layers are necessarily in direct physical contact with each other orwith the substrate, but may instead have one or more intervening layersor structures. It also does not indicate that the substrate is orientedin a manner that places the second layer physically above the firstlayer, nor that, for example, the layers are positioned over a frontface of the substrate, as that term is used herein.

In describing the embodiments illustrated in the drawings, directionalreferences, such as right, left, top, bottom, etc., are used to refer toelements or movements as they are shown in the figures. Such terms areused to simplify the description and are not to be construed as limitingthe claims in any way.

The unit symbol “μm” is used herein to refer to a value in microns. Onemicron is equal to 1×10⁻⁶ meters.

Ordinal numbers, e.g., first, second, third, etc., are used according toconventional practice, i.e., for the purpose of clearly distinguishingbetween claimed elements or features thereof. The use of such numbersdoes not suggest any other relationship, e.g., order of operation orrelative position of such elements, nor does it exclude the possiblecombination of the listed elements into a single, multiple-function,structure or housing. Furthermore, ordinal numbers used in the claimshave no specific correspondence to those used in the specification torefer to elements of disclosed embodiments on which those claims read.

Where a claim limitation recites a structure as an object of thelimitation, that structure itself is not an element of the claim, but isa modifier of the subject. For example, in a limitation that recites “aleadframe having a die paddle configured to receive a semiconductordie,” the semiconductor die is not an element of the claim, but insteadserves to define the scope of the term die paddle. Additionally,subsequent limitations in the same claim or dependent claims that reciteor characterize additional elements relative to the semiconductor die donot render the die an element of that claim.

The abstract of the present disclosure is provided as a brief outline ofsome of the principles of the invention according to one embodiment, andis not intended as a complete or definitive description of anyembodiment thereof, nor should it be relied upon to define terms used inthe specification or claims. The abstract does not limit the scope ofthe claims.

The U.S. patent application, Attorney Docket No. 850063.664, filedconcurrently herewith and by the same inventors, is directed to subjectmatter that is related to or has some technical overlap with the subjectmatter of the present disclosure, and is incorporated herein in itsentirety.

Elements of the various embodiments described above can be combined, andfurther modifications can be made, to provide further embodimentswithout deviating from the spirit and scope of the invention. Aspects ofthe embodiments can be modified, if necessary to employ concepts of thevarious patents, applications and publications to provide yet furtherembodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification, but should be construed toinclude all possible embodiments along with the full scope ofequivalents to which such claims are entitled. Accordingly, the claimsare not limited by the disclosure.

1. A semiconductor package, comprising: a package body; a die paddle ofa first metal, with a layer of a second metal, different from the firstmetal, positioned on a front surface of the die paddle and a layer of athird metal, different from the first metal, positioned on a backsurface of the die paddle, the die paddle positioned in the body so thatthe back surface of the die paddle is exposed at a back surface of thebody; a semiconductor die affixed to the front surface of the die paddleover the first layer and encapsulated within the body; and a pluralityof bond pads of the first metal, each having a layer of the second metalpositioned on a respective front surface and a layer of the third metalpositioned on a respective back surface, each being encapsulated withinthe body with a respective back surface exposed at the back surface ofthe body, and no portion exposed at a lateral face of the body.
 2. Thepackage of claim 1 wherein the die paddle and each of the plurality ofbond pads are thinner than about 100 microns.
 3. The package of claim 1wherein the die paddle and each of the plurality of bond pads arethinner than about 50 microns.
 4. The package of claim 1 wherein the diepaddle and each of the plurality of bond pads are between about 10microns and about 80 microns.
 5. The package of claim 1, comprising acontact ring of the first metal, lying in a common plane with the diepaddle and positioned to encircle the die paddle, and having a backsurface exposed at the back surface of the body.
 6. The package of claim1 wherein the second and third metals are a same metal.
 7. A leadframefor a semiconductor package, comprising: a substrate of a first metal,having first and second surfaces and extending laterally withoutsubstantial perforations over a first area; a plurality of lands coupledto the substrate on the first surface entirely within the first area;and a plurality of cavities formed in the second surface of thesubstrate without extending through the substrate, each in a positionopposite a respective one of the plurality of lands.
 8. The leadframe ofclaim 7 wherein each of the plurality of cavities has lateral dimensionsthat are less than corresponding lateral dimensions of the correspondingland opposite.
 9. The leadframe of claim 7 wherein a depth of each ofthe plurality of cavities is approximately equal to a height of each ofthe plurality of lands above the first surface.
 10. The leadframe ofclaim 7 wherein each of the plurality of lands has a face on which alayer of a second metal is deposited.
 11. The leadframe of claim 7wherein each of the plurality of cavities has an inner wall on which alayer of a second metal is deposited.
 12. The leadframe of claim 7wherein one of the plurality of lands is a die paddle, and others of theplurality of lands are bond pads.
 13. A method, comprising: etching afirst depth into a first face of a substrate and forming thereby aplurality of lands of a leadframe, the first depth being less than aninitial thickness of the substrate; and etching a plurality of cavitiesa second depth into a second face of the substrate, each positionedopposite a respective one of the lands, the second depth being less thanthe initial thickness of the substrate.
 14. The method of claim 13,comprising plating a layer of metal over the first face of thesubstrate.
 15. The method of claim 14, comprising defining lateraldimensions of each of the plurality of lands by patterning the layer ofmetal.
 16. The method of claim 13, comprising plating a layer of metalon an inner surface of each of the plurality of cavities.
 17. The methodof claim 13, comprising defining, among the lands of the leadframe, adie paddle and a plurality of bond pads.
 18. A method, comprising:affixing a semiconductor die to a first surface of a leadframe; placingcontact pads of the die in electrical contact with corresponding bondpads of the leadframe; depositing a molding compound over thesemiconductor die on the first surface of the leadframe; curing themolding compound on the first surface of the leadframe; and exposingportions of a surface of the cured molding compound by removingcorresponding portions of the leadframe, while leaving portions of theleadframe embedded in the molding compound.
 19. The method of claim 18wherein the removing comprises etching a second surface of theleadframe, opposite the first surface.
 20. The method of claim 18wherein the leaving comprises selectively etching the second surface ofthe leadframe without etching portions of the second surface over whicha metallic layer is deposited.
 21. The method of claim 18 wherein theplacing comprises placing contact pads of the die in electrical contactwith bond pads that are positioned on the first surface of the leadframesubstantially opposite corresponding portions of the second surface overwhich a metallic layer is deposited.
 22. The method of claim 18 whereinthe affixing comprises affixing the semiconductor die to a die paddlethat is positioned on the first surface of the leadframe.